A THz time domain imaging system is optimized and analyzed with ZEMAX. The requirements to the optical design of time domain imaging systems in the THz spectral region are deduced. A system is presented, which is diffraction-limited for wavelengths down to 838 μm and field points up to ±4 mm. In the optical system a 90° off-axis parabolic mirror is combined with an aspheric plastic lens. The lens was made from ZEONEX E48R®, and it was manufactured by ultraprecision machining. A resolution test of the system shows that on time domain analysis of the pulse maximum on-axis 1 LP/mm can be resolved with an intensity contrast of 0.22. The resolution of the outermost field point is 0.67 LP/mm with an intensity contrast of 0.23. An outlook of an optimized system for imaging a field of ±10 mm in x- and y-direction is given.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.